The present invention relates to an air-fuel ratio feedback control method for an internal combustion engine which is of the type that vaporized fuel is supplied from a fuel tank to the engine.
Conventionally, an air-fuel ratio feedback control method for an internal combustion engine is known, in which during operation of the engine in an air-fuel ratio feedback control region, the air-fuel ratio of a mixture supplied to the engine is controlled to a desired value by the use of a coefficient which varies with change in the output of an exhaust gas ingredient concentration sensor arranged in the exhaust system of the engine, so as to achieve excellent exhaust emission characteristics and reduce fuel consumption.
Further, an air-fuel ratio feedback control method of this type has been proposed by the assignee of the present application, e.g. in Japanese Provisional Patent Publication (Kokai) No. 60-3449, in which when the coefficient assumes a value falling outside a predetermined range, the coefficient is held at an upper or lower limit value of &he predetermined range, and then the coefficient value or upper or lower limit value is applied to the feedback control to thereby prevent excessive correction of the air-fuel ratio, and hence engine stalling, etc. due to the excessive correction when an abnormal output is produced from the exhaust gas ingredient concentration sensor due to malfunction of same.
In the meanwhile, internal combustion engines in general are equipped with a canister or the like which temporarily stores vaporized fuel produced in a fuel tank and supplies same to the engine during operation of the engine in order to prevent the vaporized fuel from being emitted into the air. When the aforesaid conventional air-fuel ratio feedback control method is applied to an internal combustion engine equipped with the canister, there arises a problem that it is impossible to attain both of the above-mentioned objects, i.e., excellent exhaust emission characteristics and reduced fuel consumption, and prevention of excessive correction of the air-fuel ratio in the event of an abnormal output of the exhaust gas ingredient concentration sensor.
More specifically, when a great amount of vaporized fuel is present in the fuel tank at a high temperature, the vaporized fuel is supplied in great quantities to the engine together with proper fuel supplied from a fuel supplying device, such as fuel injection valves, during operation of the engine, that is, the total amount of fuel supplied to the engine is temporarily increased, to overrich the air-fuel ratio of a mixture supplied to the engine. This impedes attainment of excellent exhaust emission characteristics and reduced fuel consumption. The excessive enrichment of the mixture leads to a large change in the value of the coefficient to a lean side. However, in the aforesaid conventional method, the excessive enrichment of the air-fuel ratio due to the supply of vaporized fuel was not taken into consideration. Therefore, even if the coefficient assumes a value falling outside the predetermined range not because an abnormal output is produced from the malfunctioning exhaust gas ingredient concentration sensor but because vaporized fuel is actually supplied in great quantities to the engine, the coefficient is limited to the value within the predetermined range which is constant. Consequently, the air-fuel ratio feedback control cannot be effected to such a sufficient degree as to cope with the excessive enrichment of the mixture, so that satisfactory exhaust emission characteristics and reduced fuel consumption cannot be secured.
On the other hand, if the predetermined range is set to a wider range so as for the feedback control to cope with the excessive enrichment of the mixture, it is impossible to cope with an abnormal output from the exhaust gas ingredient concentration sensor, and hence to avoid the above-mentioned problems of excessive correction of the air-fuel ratio and the resulting engine stalling.
In the meanwhile, an air-fuel ratio feedback control method for an internal combustion engine has been proposed by the present assignee, e.g. in Japanese Provisional Patent Publication (Kokai) No. 60-233328, in which during operation of the engine in an air-fuel ratio feedback control region, the air-fuel ratio of a mixture supplied to the engine is controlled by the use of either a coefficient which varies with change in the output of an exhaust gas ingredient concentration sensor arranged in the exhaust system of the engine or an average value of the coefficient.
According to this proposed control method, it is determined whether the engine is operating in a feedback control region or an operating region other than the feedback control region. When the engine is operating in the feedback control region, it is determined whether the engine is operating in an idling region or an operating region other than the idling region. When the engine is operating in the idling region, or in the operating region other than the idling region, an average value of values of the coefficient obtained in each region is calculated and stored for use in each region. When the engine has shifted to one of these operating regions within the feedback control region, the average value stored for the one region to which the engine has shifted is used as an initial value of the coefficient to thereby start the air-fuel ratio feedback control. Thus, the coefficient can be set to a proper initial value at the start of the feedback control, whereby the accuracy of the feedback control is improved.
However, if this method is applied to the abovedescribed type of internal combustion engine to which vaporized fuel is supplied by way of a canister or the like during operation of the engine, there arises a problem that the air-fuel ratio is leaned when the engine has shifted to the feedback control region, which can even cause engine stalling.
More specifically, in order to cope with the aforementioned excessive enrichment of the air-fuel ratio caused by the supply of vaporized fuel to the engine, the coefficient or the average value thereof is automatically shifted to a value which leans the mixture, even if the engine is operating in the same region within the feedback control region. As a result, in the case where a great amount of vaporized fuel was present at the end of the immediately preceding operation of the engine in a region within the feedback control region, and a small amount of vaporized fuel is present when the engine has returned to the same region within the feedback control region, the above-mentioned average value obtained during the immediately preceding feedback control operation, which has been shifted to a value leaning the mixture due to the supply of vaporized fuel to the engine, is used as an initial value of the coefficient, which results in an overlean air-fuel ratio of the mixture, and even in engine stalling if the degree of leaning of the mixture is great.